AE vs Single Target Burns, let's look at a typical party's pull of trash mobs in AK:

Mob 1 - 100 DPS - 20k hp
Mob 2 - 125 DPS - 20k hp
Mob 3 - 100 DPS - 20k hp

If we AE them down, and the party is doing, let's say 500dps total between all mobs, we are left with:
120 seconds of 325 DPS (100+125+100) = 39,000 damage dealt to the tank.

Now, let's say we single target burn with marks, we're doing the same dps, but one at a time:
Mob1 - up for 120s - produces 100dps = 12,000 damage produced
Mob3 - up for 80s - produces 100dps = 8,000 damage produced
Mob2 - up for 40s - produces 125dps = 5,000 damage produced
Total damage produced: 25,000

So we now have about 64% of total damage produced. That's literally the most powerful debuff I have ever heard of in this game. Now, if you are overgeared it matters so little and the time is the same without the pesky targeting. If you are a fledgling group or you have low tankage/healing power, marking REALLY helps.